function equalsFn(a, b) {
	return a === b
}
class Node {
	constructor(element) {
		this.element = element
		this.next = undefined
	}
}

export default class LinkedList {
	#count = 0
	#head = undefined
	#equalsFn = equalsFn
	push(element) {
		const node = new Node(element)
		let current
		if (this.#head == null) {
			this.#head = node
		} else {
			current = this.#head
			while (current.next != null) {
				current = current.next
			}
			current.next = node
		}
		this.#count++
	}

	removeAt(index) {
		if (index >= 0 && index < this.#count) {
			let current = this.#head
			// 移除第一项
			if (index === 0) {
				this.#head = current.next
			} else {
				let previous
				for (let i = 0; i < index; i++) {
					previous = current
					current = current.next
				}
				previous.next = current.next
			}
			this.#count--
			return current.element
		}
		return undefined
	}
	remove(element) {
		const index = this.indexOf(element)
		return this.removeAt(index)
	}
	getElementAt(index) {
		if (index >= 0 && index <= this.#count) {
			let node = this.#head
			for (let i = 0; i < index && node !== null; i++) {
				node = node.next
			}
			return node
		}
		return undefined
	}
	insert(element, index) {
		if (index >= 0 && index <= this.#count) {
			const node = new Node(element)
			if (index === 0) {
				const current = this.#head
				node.next = current
				this.#head = node
			} else {
				const previous = this.getElementAt(index - 1)
				const current = previous.next
				node.next = current
				previous.next = node
			}
			this.#count++
			return true
		}
		return false
	}
	indexOf(element) {
		let current = this.#head
		for (let i = 0; i < this.#count && current != null; i++) {
			if (this.#equalsFn(element, current.element)) {
				return i
			}
			current = current.next
		}
		return -1
	}
	size() {
		return this.#count
	}
	isEmpty() {
		return this.size() === 0
	}
	getHead() {
		return this.#head
	}
	toString() {
		if (this.#head == null) {
			return ''
		}
		let objString = `${this.#head.element}`
		let current = this.#head.next
		for (let i = 0; i < this.size() && current != null; i++) {
			objString = `${objString},${current.element}`
			current = current.next
		}
		return objString
	}
}
